Features.yml: add screenshots

Add screenshots to features in features.yml.

Closes #1605 (closed)

Artboards w 1235 px x h 822.85 px

Notes:

• created this https://gitlab.com/gitlab-org/gitlab-ce/issues/36766 to fix the commit page dropdown menu, but already fixed on the screenshot added here.
• this branch was created from release-9-5 (!6668 (merged)) to avoid conflicts, so the release post should be merged before this MR.